In known water filtration apparatus the water to be filtered is fed to a tank, and filters down under gravity through a filter medium, from where it is collected. An apparatus typically has several adjacent tanks, each connected to a main enclosed conduit, which collects the filtered water. The filter medium in each tank rests on underdrain blocks, with the filtered water passing through the blocks into the conduit. The blocks are arranged in rows, known as laterals, accommodated on the floor of a tank, and extending laterally away from the conduit. Each lateral is connected to the conduit through appropriate apertures in the tank wall to allow the filtered water to pass into the conduit, and also to allow reverse gas and liquid flow for a backwash operation which cleans the filter medium.
In a backwash operation the underdrain blocks provide the correct distribution of gas and liquid. A backwash operation typically first pumps gas (usually air) to the filter medium to release solids, then gas and liquid (usually water) concurrently, the gas to continue release of solids and the liquid to move them out of the filter medium, and lastly just the liquid to complete the removal of the solids. In order to ensure that the backwash operation is efficient, the correct amounts of gas and liquid should be provided to the blocks, and the block which is adjacent the apertures should seal against the tank wall to prevent leakage of gas.